Characteristics, properties, and potential applications of circulating cell-free dna in clinical diagnostics: a focus on transplantation.

The initial discovery of cell-free DNA (cfDNA) in 1948 by Mandel and Metais has led to numerous investigations evaluating the role of cfDNA in various disease states. cfDNA has been characterized in various patient populations with similar results. cfDNA are typically 150 bp of double-stranded DNA that are thought to be released from nucleosomes during apoptosis and necrosis. They are found in circulation as monomers, dimers, and trimers. Different specimen types yield significantly different amounts cfDNA. While serum yields the highest amount of cfDNA, it contains the most genomic DNA contamination compared to Streck and plasma specimen types. The utility of cfDNA as a biomarker was advanced by the completion of the Human Genome Project and enabled interrogation of tumor markers in cancer patients. While tumor genetics may have been the initial application of cfDNA, the most successful application of cfDNA as a clinical biomarker is noninvasive prenatal testing (NIPT). CfDNA has become the gold-standard for NIPT testing, allowing for high sensitivity while maintaining specificity for aneuploidy. Because prenatal testing is essentially mixed genome analysis, application of cfDNA analysis to solid organ transplantation is a clear diagnostic target. There have been several studies examining the role of cfDNA in solid organ transplantation. These studies identified cfDNA as a surrogate marker for rejection with a high level of concordance with biopsies. While the data thus far are promising, there is still a need for more prospective studies to determine the clinical utility of cfDNA in solid organ transplant rejection.